1. Technical Field
This invention relates generally to the field of automated storage libraries and more particularly, to a system and method for sequencing commands to a tape cartridge transporter for efficiently moving tape cartridges between storage racks and drive units for cartridge playback and recording.
2. Description of the Related Art
Numerous business applications require very large databases for storing information used in the business on a continuous basis. Database storage requirements may exceed 100s or 1,000s of gigabytes of data. Data storage costs generally limit such large databases to being stored on magnetic tape, which presently offers the lowest data storage costs. A premium is placed upon speed and reliability of recovery of data, particularly when relatively frequent access to the information stored within the database occurs. However, tape storage generally requires the longest access time for retrieving the stored data of the major data storage systems. For example, once a tape is loaded onto a tape drive, accessing the data stored on the loaded tape is substantially slower than accessing data stored on a direct storage access device (DASD) (e.g. a disk drive). Substantial delay is also incurred in locating and mounting the desired volume, be it on tape, magnetic disk or optical disk. Automated storage libraries offer improved speed and reliability for storing and retrieving data stored on storage media, including magnetic tape. Early magnetic tape databases required an operator to retrieve a tape from a shelf and load the tape on a tape drive. This was very slow and prone to operator mistakes. Also, early magnetic tape reels were relatively large, i.e., 10.5 inch diameter reels, and furthermore, only 9 tracks were available for writing data to the tape. Correspondingly large areas were necessary to store the many required 10.5 inch magnetic tapes. Newer magnetic tapes are protected by a cartridge housing, and are substantially smaller than the 10.5 inch reels. Additionally, data can now be written to 36 tracks or more. Hence, each magnetic tape cartridge can store many times more data than the older reels. The advances made in magnetic tape technology have made automated cartridge libraries more effective since smaller areas are required to store more data on lighter weight cartridges.
Access time and reliability is improved by automatically managing the storage and retrieval of tape cartridges. Automated cartridge libraries include a plurality of storage slots for storing library resident tape cartridges, a robotic picker mechanism, and one or more tape drives. The robotic picker operates on command to transfer a tape cartridge between a storage slot and a tape drive within seconds. Operational benefits of using an automated cartridge library include: greater reliability in tape cartridge mounts; better predictability in request-to-mount time; and improved off-shift availability. An input/output port is also provided on the automated cartridge library for inserting and exiting cartridges from the library.
Further reduction in time between request and response may be had by efficient utilization of the resources of the automated library. Efficient utilization demands first that mounts and demounts of tape cartridges be avoided and second, that when mounts and demounts are unavoidable, that they be carried out expeditiously. Reduction in the frequency of mounts and demounts is achieved through organization of data requests to minimize the demand for tape mount and demount operations. An example of such a system is disclosed in U.S. Pat. No. 4,876,662, to Pence, which is assigned to the assignee of this patent. The U.S. Pat. No. '662 patent also discusses algorithms for selecting appropriate levels in an hierarchical storage for storage of data. The second route to improving efficiency in utilization of an automated library is through efficient dispatch of the robotic picker mechanism to perform outstanding requests for tape mounts and demounts.